Reticle transmittance measurement method, projection exposure device, and projection exposure method

ABSTRACT

When a reticle is used first, the reticle is actually loaded in a projection exposure device and measured by one of oblique measurement and random measurement, thereby avoiding the fear of uneven sampling and determining the reticle transmittance of the entire reticle as the parent population, without increasing the, sampling count. The same effect can be obtained by making the measurement spot size, which is fixed in general, variable and by changing the angle of incidence in relation to the measurement spot size.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of measuring a transmittanceof a reticle that is used in manufacture of a semiconductor device, anda projection exposure device (projection aligner) and a projectionexposure method that are used in the measurement.

2. Description of the Related Art

In a projection exposure device, for example, a stepper, when a reticleis first used, the reticle is actually loaded in the device to beexposed to light from a mercury lamp serving as a light source, and(transmitted energy/incident energy) is calculated as a reticletransmittance. Sampling is carried out through the exposure at an equalinterval over the entire reticle patterns. Then estimation oncharacteristics is required over the entire reticle viewed as a parentpopulation. However the sampling tends to be uneven since it isdifficult in practice to recognize what pattern is there on the reticle.For example, in the case of a pattern that is repeated at the same pitchas that of the sampling, the result calculated as the reticletransmittance is far from the real value. A correction function isactivated in projection exposure devices when the amount of transmittinglight increases, in order to cancel out the effect of lens expansionfrom heat that is generated by the increased exposure load. However,incorrect feedback leads to a focus shift, causing an increase in linewidth fluctuation and a failure in maintaining the rectangular resistprofile, which deteriorates formation of a desired pattern. Shorts andopens consequently develop in the wiring pattern, thereby impairingquality.

Increasing the sampling count has therefore been tried in an attempt tocapture the entire picture of the reticle as the parent population.However, the fear of uneven sampling remains in the case of equalinterval measurement and, even if accurate measurement is somehowmanaged, the vast amount of time required in this approach impairs theproductivity of the projection exposure device.

As described in the description of the related art, the problem of amismatch between the result acquired as the reticle transmittance andthe real value may occur when a pattern repeated at the same pitch asthat of the sampling is included. In the case where a reticletransmittance that takes into account the characteristics of patterns isdetermined, for example, one method of determining the reticletransmittance, when a different shot has a different area as in JapanesePublished Patent Application No. 2001-297961, involves conducting totallight amount measurement on the entire reticle surface, storing dataabout a projected image, and calculating an actually exposed portion ofthe reticle from the stored data and from the opening/closing of amasking blade. A prerequisite for the calculation of an actually exposedportion of the reticle is that accurate total light amount measurementon the entire reticle surface can be conducted first. When a patternrepeated at the same pitch as that of the sampling is included, a resultcalculated as the reticle transmittance does not match reality, whichmakes it difficult to calculate an actually exposed portion of thereticle from the opening/closing of the masking blade.

Increasing the sampling count in an attempt to capture the entirepicture of the reticle as the parent population is a way to measure thereticle transmittance accurately. Another method of determining thereticle transmittance is described in Japanese Published PatentApplication No. H06-236838, in which a reticle is actually loaded in aprojection exposure device to be exposed to light from a mercury lampserving as a light source, and image data of a transfer image of areticle pattern formed by the exposure is input to be used in thecalculation of the reticle transmittance. However, both methods have aproblem in that the huge sampling count requires a vast amount of timefor measurement and accordingly impairs the productivity of theprojection exposure device.

SUMMARY OF THE INVENTION

The present invention has been made in view of those problems, and it isan object of the present invention to provide, as a countermeasure for afocus shift that is caused by lens expansion due to increased exposureload in a projection exposure device, a novel measurement method and anovel projection exposure device that are applicable to reticletransmittance measurement for the calculation of a load used in thecorrection of exposure load.

In order to attain this object, a semiconductor device exposure methodaccording to one embodiment of the present invention uses the followingmeasures.

When a reticle is used first, the reticle is actually loaded in aprojection exposure device and measured by one of oblique measurementand random measurement, thereby avoiding the fear of uneven sampling andcapturing the entire picture of the reticle as the parent population,without increasing the sampling count. Results acquired as the reticletransmittance in this manner closely matches reality. In addition, themeasurement spot size, which is fixed in the related art, may be madevariable in the one embodiment of the present invention so that the sameeffect can be obtained by changing the angle of incidence in relation tothe measurement spot size.

The reticle transmittance of the entire reticle is determined bymeasuring, in sampling, the transmittance of at least one chip area in amulti-patterned reticle that has a plurality of identical chips.

In order to determine the reticle transmittance in a short time withoutactually measuring the reticle transmittance, by making a different useof design data of a reticle pattern, there is employed an exposuremethod including: creating design data of a reticle pattern with the useof a standard CAD tool; executing data conversion in which the designdata is converted into a standard file format such as the GDS II streamformat or the CIF format, in a manner written by mask CAD; determiningthe reticle transmittance from the converted design data; and saving thedetermined reticle transmittance. Further, there are employed asemiconductor exposure device and exposure method for determining thereticle transmittance directly from data, without measuring the reticletransmittance of an actual reticle.

According to the one embodiment of the present invention, there may beprovided, as a countermeasure for the focus shift that is caused by lensexpansion due to increased exposure load in the projection exposuredevice, a method of determining the reticle transmittance with highprecision in a short time no matter what characteristics a pattern has,that is, a method of determining an accurate reticle transmittancewithout impairing productivity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a stepper according to an embodiment of thepresent invention.

FIG. 2 is a block diagram of a stepper of the related art.

FIG. 3 is an explanatory diagram for illustrating a method of measuringthe reticle transmittance by oblique measurement.

FIG. 4 is an explanatory diagram for illustrating a method of measuringthe reticle transmittance by random measurement.

FIG. 5 is an explanatory diagram for illustrating a method of angledmeasurement that is suited to a small measurement spot size.

FIG. 6 is an explanatory diagram for illustrating a method of angledmeasurement that is suited to a large measurement spot size.

FIG. 7 is an explanatory diagram for illustrating a method of measuringthe transmittance of one chip area.

FIG. 8 is an explanatory diagram for illustrating a method of measuringthe transmittance of four chip areas.

FIG. 9 is an explanatory diagram for illustrating a method of measuringonly the transmittance of arbitrarily selected areas.

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 1 is a block diagram of a stepper, which is one of projectionexposure devices, according to an embodiment of the present invention.The stepper has a function of correcting exposure load based on designdata. The stepper includes an illumination optical system 1 a reticle 2,which is an original plate for actually measuring the reticletransmittance, a projection optical system 3 for transferring byexposure a desired reticle pattern that is reduced in size to, forexample, ⅕ onto a wafer, a stage 4 for moving the wafer to a givenmeasurement spot where the reticle transmittance is measured, a chuck 5,which supports the wafer, a photodetector 6, which measures the amountof light passed through the projection optical system, and a CPU 7,which corrects exposure load based on a measured reticle transmittance,which calculates the reticle transmittance from design data to correctexposure load, and which controls the driving of the illuminationoptical system 1 and of the stage 4.

A reticle transmittance storage device 8 is a storage device for savingdata of an actually measured reticle transmittance and data of a reticletransmittance that is calculated from design data 9. The method thatuses the design data 9 is described later.

For reference, a block diagram of a stepper of the related art isillustrated in FIG. 2. An apparent difference is that the stepper of therelated art does not involve associating the reticle transmittance withdesign data, and a reticle transmittance storage device 8 of the relatedart does not make use of data of a reticle transmittance that iscalculated from design data.

With use of the above-mentioned structure, when a pattern repeated atthe same pitch as that of the sampling, the problem of the related art,in which a result calculated as the reticle transmittance does not matchreality, may be solved. A specific description is given below on methodsof measuring the reticle transmittance.

A first method involves, as illustrated in FIG. 3, putting a lightamount measurement spot 10 on an oblique route that runs diagonallyrelative to the reticle, for example, and measuring at a 0.2-mm pitch,as opposed to the related art where measurement is made at a 0.2-mmpitch in an X-direction and a Y-direction, for example. The meaning ofan oblique route running diagonally relative to the reticle is a routerunning along a straight line that is slanted relative to the four sidesof the reticle and that does not usually match any diagonal line of thereticle. The measurement spot in this case is set so that the samplingin the X-direction and the Y-direction is conducted at a pitch differentfrom the repetition pitch of a reticle pattern. The transmittance mayalso be measured by putting the measurement spot 10 on an oblique routethat runs diagonally relative to the reticle and switching the pitch,for example, from 0.2 mm to 0.3 mm, back to 0.2 mm, and then to 0.3 mm.In this case too, the measurement spot is set so that the sampling inthe X-direction and the Y-direction does not match with a reticlepattern having the same repetition pitch as the sampling pitch andmatches with a reticle pattern having a different pitch.

A reticle transmittance that represents the overall characteristics ofthe reticle is determined from the sampling described above, and issaved in the reticle transmittance storage device 8. Based on thedetermined reticle transmittance, the CPU (exposure load correctingdevice) 7 of FIG. 1 corrects exposure load by correcting focus, lensdistortion, and magnification in combination as needed, and sendsfeedback to the projection optical system 3 of FIG. 1 to performprojection exposure.

A second method involves, as illustrated in FIG. 4, giving the pitch ofthe measuring spot 10 a range of 0.2 mm to 1.0 mm, for example, asopposed to the related art where measurement is made at a 0.2-mm pitchby moving measurement spots in order in the X-direction and theY-direction, for example. In the second method, the measurement spots,which are moved individually in the X-direction and the Y-direction inthe related art, are combined and moved randomly in order to performmeasurement with no regularity, and the sampling operation is set so asto match not with a reticle pattern that has the same repetition pitchas the sampling pitch but with a reticle pattern that has a differentpitch.

A third method involves, as illustrated in FIG. 5, making the diameter(Φ) of the measurement spot variable between 0.3 mm and 1.0 mm andchanging the angle of inclination to suit the measurement spot size, asopposed to the related art where the diameter (Φ) of the measurementspot is fixed at 0.3 mm, for example. When the size of a measurementspot 11 viewed from the front is small, for example, when themeasurement spot 11 is 0.3 mm in diameter (Φ), the reticle transmittanceis measured by setting a small angle between 10 degrees and 30 degreesto an inclination angle θ1 (12) with respect to a surface of the reticle2 viewed edge-on. This secures a large effective measurement areadespite the small size of the measurement spot. When the size of ameasurement spot 13 viewed from the front is large, for example, whenthe measurement spot 13 is 1.0 mm in diameter (Φ)as illustrated in FIG.6, on the other hand, the reticle transmittance can be measured bysetting a large angle between 70 degrees and 90 degrees to aninclination angle θ2 (14) with respect to the surface of the reticle 2viewed edge-on. This is because a large measurement area can be securedwhen the measurement spot size is large. Varying the measurement spotsize in this manner prevents sampling that is conducted at the samepitch as the repetition pitch of a reticle pattern.

A fourth method involves, as illustrated in FIG. 7, measuring thetransmittance by sampling only one chip area in a multi-patternedreticle that includes a plurality of identical chips, and calculatingthe reticle transmittance from the result of the measurement. The onechip area, which is denoted by 15, is, for example, a unit areasurrounded by an area that reaches the center (midpoint) of scribe linesalong which the wafer is cut in dicing, inside and outside the operatingarea of a semiconductor device. The entire multi-patterned area is theentire reticle surface, which includes all these unit areas. Parametersfor these are input to the projection exposure device in advance as asampling area and the entire multi-patterned area. The reticletransmittance for the entire reticle is determined by calculating anarea dimension ratio of the one chip area 15 and the entiremulti-patterned area, and is saved in the reticle transmittance storagedevice 8. Based on the determined reticle transmittance, the CPU(exposure load correcting device) 7 of FIG. 1 corrects exposure load bycorrecting focus, lens distortion, and magnification in combination asneeded, and sends feedback to the projection optical system 3 of FIG. 1to perform projection exposure.

A fifth method involves, as illustrated in FIG. 8, measuring thetransmittance by sampling only four chip areas 16 in a multi-patternedreticle that includes a plurality of identical chips, and calculatingthe reticle transmittance from the result of the measurement. Thereticle transmittance of the entire reticle surface is determined bycalculating an areal dimension ratio of the four chip areas 16 and theentire multi-patterned area, and the same exposure load correction as inthe fourth method is thus accomplished.

A sixth method involves, as illustrated in FIG. 9, measuring thetransmittance by sampling only a selected area 17, which is ¼ of theentire reticle in a multi-patterned reticle that includes a plurality ofidentical chips, and calculating the reticle transmittance from theresult of the measurement. The reticle transmittance of the entirereticle surface is determined by calculating an areal dimension ratio ofthe selected area 17, which is ¼ of the entire reticle, and the entiremulti-patterned area, and the same exposure load correction as in thefourth and fifth methods is thus accomplished while reducing ameasurement time to ¼ of a normal measurement time.

A seventh method differs from the methods described above, anddetermines the reticle transmittance without actually measuring thetransmittance. In order to obtain the design data 9 of FIG. 1, designdata of a reticle is created first for all layers by CAD. Dataconversion is executed next to convert the design data into a standardfile format such as the GDS II stream format or the CIF format. Theconverted data and the areal dimensions that a light shielding filmtakes up on the reticle are used to determine the reticle transmittance.The determined reticle transmittance is saved as data in the reticletransmittance storage device 8 of FIG. 1 via a local area network (LAN).Based on the reticle transmittance saved as data, the CPU (exposure loadcorrecting device) 7 of FIG. 1 corrects exposure load by correctingfocus, lens distortion, and magnification in combination as needed, andsends feedback to the projection optical system 3 of FIG. 1. The reticletransmittance can be determined in a short time without measuring thetransmittance of an actual reticle in this manner. A comparison of thereticle transmittance thus determined without measuring with the reticletransmittance that is determined through measuring by one of the firstto sixth methods is useful in improving the precision of determining thereticle transmittance from CAD data or in accomplishing appropriatesampling in measurement, which further improves precision.

The present invention is applicable to the manufacture of a device thatrequires micro-fabrication in which a projection exposure device uses areticle and photolithography technology in one of processes, such as asemiconductor substrate or a MEMS.

What is claimed is:
 1. A reticle transmittance measurement method for aprojection exposure device configured to project a pattern of a reticleonto a wafer to obtain a given pattern on the wafer, the reticletransmittance measurement method comprising: sampling a transmittance ofthe reticle by measuring the transmittance of the reticle along anoblique route that runs diagonally relative to the reticle, with asampling pitch being set so as to match with a different pattern of areticle pattern that has a same repetition pitch, and not to match witha same pattern of the reticle pattern that has the same repetitionpitch.
 2. A reticle transmittance measurement method for a projectionexposure device configured to project a pattern of a reticle onto awafer to obtain a given pattern on the wafer, the reticle transmittancemeasurement method comprising: determining sampling pitches in anX-direction and a Y-direction to have a random value within apredetermined range instead of having a fixed value, to thereby set thesampling pitches so as to match with a different pattern of a reticlepattern that has a same repetition pitch, and not to match with a samepattern of the reticle pattern that has the same repetition pitch.
 3. Areticle transmittance measurement method for a projection exposuredevice configured to project a pattern of a reticle onto a wafer toobtain a given pattern on the wafer, the reticle transmittancemeasurement method comprising performing sampling to measure atransmittance of at least one chip area in a multi-patterned reticlethat includes a plurality of identical chips, to thereby calculate atransmittance of the reticle.
 4. A reticle transmittance measurementmethod according to claim 3, wherein the at least one chip areacomprises a unit area including an operating area of a semiconductordevice and surrounded by an area that reaches a center of scribe linessurrounding the operating area.
 5. A projection exposure method,comprising performing exposure load correction based on a reticletransmittance that is determined by the reticle transmittancemeasurement method of claim
 1. 6. A projection exposure method accordingto claim 5, wherein the exposure load correction comprises at least oneof focus correction, lens distortion correction, or magnificationcorrection.